The turnover rate of gamma-aminobutyric acid (GABA) has been measured in n. accumbens, n. caudatus, globus pallidus, substantia nigra, septum and tuberculum olfactorium where the pool of glutamate functions primarily as a GABA precursor. It has not been possible to estimate GABA turnover in areas where glutamate is a primary neurotransmitter such as deep cerebellar nuclei, cerebellar cortex, colliculus, hippocampus or in areas of the cortex (frontal, parietal, occipital or temporal). GABA turnover studies have shown that the glutamatergic collaterials from the hippocampal pyramidal cells projecting to the lateral septum do not control the GABAergic interneurons located here. However, various transmitter systems trans-synaptically modulate the GABAergic neurons located in various brain nuclei. Blockade of dopamine receptors with haloperidol increases the GABA turnover in accumbens and globus pallidus. Opiate receptor agonists and cholinergic agonists increase the GABA turnover in globus pallidus. These studies suggest that GABA turnover is a valuable tool in the study of modulation of GABA neurons by various neuronal inputs.